A “satellite navigation system” is understood to mean any system devoted to wide area navigation, such as for example existing GNSSs (Global Navigation Satellite Systems) called GPS, GLONASS or the future GALILEO system, and all their equivalents and derivative systems.
A person skilled in the art is well aware of the location principle of satellite navigation systems. For example, in the GPS system, the radiofrequency signal transmitted by a satellite is encoded, and the time taken by this signal to reach the receiver to be located is used to determine the distance between this satellite and this receiver, this distance being preferably called the pseudorange.
To improve existing satellite systems in terms of accuracy, integrity, continuity and availability, augmentation systems have been provided. Integrity performance is particularly important since it plays a part in applications on which the safety of users depends. The European satellite augmentation system EGNOS improves the performance of two satellite systems, GPS and GLONASS. It transmits integrity messages to the user, which allow the user to thus assess the confidence they can have in the coordinates of their position and in the end act accordingly. EGNOS transmits pseudorange corrections and their accuracy in the form of a standard deviation and corrects:                errors related to the passage of electromagnetic waves in the ionosphere and troposphere;        errors related to GPS and geostationary satellites (slow corrections: orbit errors and clock errors);        errors that vary rapidly such as clock errors due to “Selective Availability SA” (fast corrections).        
Mention may be made of data called SREW (Satellite Residual Error for the Worst user location) as calculated pseudorange error data. This data represents the orbit error and clock error of the satellite seen from the worst-case user in the service area. UDRE (User Differential Range Error) data is an upper-bound estimate of the SREW. Mention may also be made of ionospheric errors: GIVD (Grid Ionospheric Vertical Delay). The ionospheric layer has been divided up with the aid of a grid. For each point on the grid, an estimate of the associated ionospheric delay is transmitted. The user whose measurement has little chance of piercing a grid point exactly will interpolate the values supplied for each of the four grid points neighboring the pierce point of the user. Moreover, the user will not necessarily see the satellite vertically, but will most certainly make an oblique measurement.
Only the sources of errors related to the receiver (clock error, eccentricity, multiple paths) persist. The user then calculates the “augmented” position, i.e. a position that is improved by virtue of the pseudorange corrections. The accuracy of this position is assessed by comparing it with a reference position. The pseudorange corrections allow the user to calculate in real time the accuracy of their position by error propagation. In civil aviation for example, protection levels are deduced from the positional accuracy. These protection levels are strict confidence intervals. These protection levels must not exceed the alert level specified for the flight phase. The integrity, availability and continuity of a navigation system are assessed with the aid of the position error, protection levels and alert levels.
FIG. 1 describes an architecture of a satellite navigation system including a GNSS differential positioning system and SBASs (Satellite Based Augmentation Systems) and GBASs (Ground Based Augmentation Systems). The aircraft take receivers on board which are connected to the augmentation systems.
EGNOS is an SBAS type system including a ground infrastructure SBAS G and geostationary satellites SBAS S. The ground infrastructure comprises a plurality of receiving stations distributed over a wide geographical area, which receive data from GNSS satellites and determine the pseudoranges, and a central station 1 for control and processing, which determines, from the pseudoranges transmitted by the SBAS G receiving stations, corrections and integrity which are combined into one SBAS signal. The geostationary satellites relay this signal from the central station to the receivers on the aircraft.
The GBAS system includes ground beacons intended to respond to local requirements necessitating a much higher level of accuracy within a determined operating range. These beacons are for example located in airport areas. The GBAS system also includes receivers fitted on board aircraft. The GNSS system supplies the aircraft and ground beacons information to calculate pseudoranges. The ground beacon supplies pseudorange correction information and information on the integrity of the differential positioning for each GNSS satellite in sight. The GBAS beacons supply more accurate corrections than those of an SBAS central station. In addition, the GBAS beacons are under the authority of the air control service which can thus control the transmission of these beacons according to the positional accuracy and integrity required.
A number of solutions for detecting the non-integrity of satellite systems are known, but none is capable of providing an indication of the integrity of the system in real time for very low-probability events, i.e. of the order of around 10−7. By way of example, mention may be made of U.S. Pat. No. 7,089,452 B2 describing a technique for evaluating the integrity of the GPS satellite system based on an estimator using the technique of moments. The current techniques are capable only of determining whether or not the satellite systems meet the certification. They perform only an a posteriori check of the level of integrity of the system. The main disadvantage of this type of solution is that an operator can only deactivate the system once the critical threshold is exceeded. These techniques do not provide for checking for the change of state of integrity of a satellite navigation system and in the end anticipating a failing situation.
It is known that augmented satellite systems are capable of meeting the specifications required for very low-probability events. These verifications have been carried out through cumbersome and tedious processes during the development phases. Once placed in operation, it is no longer possible to carry out these checks. According to the current techniques, they would require measurements to be carried out for which the test duration would be almost infinite. Specifically, to carry out integrity margin measurements, conventional inferential statistics attempts to model the behavior of a random variable on the observable domain of implementations. To obtain relevant statistics, it is necessary to retrieve sufficiently decorrelated data so as not to measure redundant information. It is estimated that sampling with a period of about five minutes between each measurement is required. Now, given the low probability of events that one seeks to detect, this would involve gathering billions of samples over thousands of years of measurements.
In addition, satellite systems have been certified at a level of integrity at 10−7 for the transmission of information over the whole of the satellite system and for a particular system. The current techniques do not provide for having a measurement of integrity at 10−7 over the whole of the life cycle of the satellite system and do not take into account specific disruptive elements at each location.